This invention relates to an X-ray illumination optical system and an X-ray reduction exposure apparatus using the same and, in another aspect, it relates to a device manufacturing method using such an X-ray reduction exposure apparatus.
An X-ray reduction projection exposure process is used for the manufacture of microdevices such as semiconductor circuit devices, having fine patterns. In such a process, a mask having a circuit pattern formed thereon is illuminated with X-rays and an image of the pattern of the mask is projected, in a reduced scale, on the surface of a wafer. A resist on the wafer surface is sensitized, whereby the pattern is transferred and printed thereon.
FIG. 12 is a schematic view of a known example of an X-ray reduction projection exposure apparatus, FIG. 13 is a schematic and perspective view of a reflection type integrator, and FIG. 14 is a schematic view for explaining an illumination region upon the surface of a mask. In these drawings, denoted at 1001 is a light emission point for X-rays, and denoted at 1002 is an X-ray beam. Denoted at 1004 is a first rotational parabolic surface mirror, and denoted at 1005 is a reflection type integrator. Denoted at 1006 is a second rotational parabolic surface mirror, and denoled at 1007 is a mask. Denoted at 1008 is a projection optical system, and denoted at 1009 is a wafer. Denoted at 1010 is a mask stage, and denoted at 1011 is a wafer stage. Denoted at 1012 is an arcuate aperture, and denoted at 1013 is a laser light source. Denoted at 1014 is a laser collecting optical system, and denoted at 1015 is an illumination region defined on the surface of the mask. Denoted at 1016 is an arcuate region within which the exposure is performed.
An X-ray light source may comprise a laser plasma or an undulator. In the illumination optical system, X-rays from the light source are collected by means of the first rotational parabolic surface mirror 1004, and the collected X-ray beam is projected on the reflection type integrator 1005, whereby secondary light sources are formed. X-rays from these secondary light sources are collected by means of the second rotational parabolic surface mirror 1006, to illuminate the mask 1007.
The mask 1007 comprises a multilayered film reflection mirror on which non-reflective portions are defined by use of an X-ray absorptive material, for example, whereby a transfer pattern is formed thereon. X-rays reflected by the mask 1007 are imaged by the projection optical system 1008 upon the surface of the wafer 1009. The projection optical system 1008 is designed so as to provide good imaging performance with respect to a narrow arcuate region off the optical axis. In order that the exposure is performed only by use of this narrow arcuate region, an aperture 1012 having an arcuate opening is disposed just before the wafer 1009. For the pattern transfer to the whole surface of the mask having a rectangular shape, the exposure process is performed while scanningly moving the mask 1007 and the wafer 1009 simultaneously.
The reflection type integrator 1005 may comprise a fly""s-eye mirror having a number of small spherical surfaces arrayed two-dimensionally, as is best seen in FIG. 13, which are adapted to define a number of secondary light sources.
Here, if the spatial extension of the secondary light source group is d, the angular extension of X-rays emitted from each secondary light source is xcex8, and the focal length of the second parabolic surface mirror 1006 is f, then the size of the illumination region 1015 on the mask 1007 surface is fxc3x97xcex8 and the angular extension of X-rays illuminating a single point on the mask is d/f.
As a parameter which represents the characteristic of the illumination optical system, there is a coherence factor "sgr". If the mask-side numerical aperture of the projection optical system 1008 is NAp1, and the mask-side numerical aperture of the illumination optical system is NAi, the coherence factor can be defined as follows:
"sgr"=NAi/NAp1
The optimum value of "sgr" is determined by the required resolution and contrast. Generally, if the factor "sgr" is too small, an interference pattern appears at the edge portion of an image of a fine pattern as projected on the wafer 1009. If the factor "sgr" is too large, the contrast of the projected image reduces.
If "sgr" is zero, it is called coherent illumination. Regarding the transfer function OTF of the optical system, a constant value will be shown up to a particular spatial frequency as can be given by NAp2/xcex where NAp2 is the wafer-side numerical aperture of the projection optical system and xcex is the wavelength of the X-ray beam. For a higher frequency above the particular frequency, it becomes equal to zero, and resolving is not attainable.
If, on the other hand, "sgr" is equal to 1, it is called incoherent illumination. The transfer function OTF reduces with an increase in the spatial frequency, but it does not become equal to zero unless a particular spatial frequency given by 2xc3x97NAp2/"sgr" is reached. Thus, a more fine pattern can be resolved. In X-ray exposure, an optimum value of "sgr" may be selected in accordance with the shape or size of a pattern to be transferred, or the characteristic of a resist process to be adopted. Usually, a value such as "sgr"=0.1-1.0 may be set.
There is a problem to be solved, in conventional X-ray reduction projection exposure apparatuses. That is, as shown in FIG. 14, the illumination region 1015 on the mask surface has a rectangular shape or an elliptical shape, including an arcuate region 1016 through which the exposure is actually made. Thus, the region outside the exposure region is illuminated with many X-rays. These X-rays are not contributable to the exposure process, and they are wasteful. The loss of X-ray light quantity is large and it leads to prolongation of the exposure time. Thus, the throughput is low.
It is accordingly an object of the present invention to provide an X-ray reduction exposure apparatus and/or an X-ray illumination optical system therefor, by which the loss of light quantity can be very small, the exposure time can be shortened, and the throughput can be improved.
It is another object of the present invention to provide a device manufacturing method based on such an X-ray reduction exposure apparatus.
In accordance with an aspect of the present invention, there is provided an X-ray illumination optical system, comprising: a reflection type integrator having cylindrical surfaces, for reflecting an X-ray beam; and a concave mirror for reflecting the X-ray beam reflected by said integrator and for illuminating an object with the X-ray beam.
The system may further comprise a second concave mirror for projecting a parallel X-ray beam onto said integrator.
Said integrator may provide a secondary light source and said concave mirror may have a focal point which is disposed at the position of said secondary light source.
Said concave mirror may have a reflection surface of a rotational parabolic shape.
An axis of said cylindrical surface of said integrator and an axis of the X-ray beam impinging on said integrator may be placed on the same plane.
There may be an illumination region defined on the object, having an arcuate shape.
Said integrator may have a reflection surface on which a multilayered film is formed.
The illumination of the object may substantially satisfy the conditions for Koehler illumination.
The X-ray beam may comprise one of a beam emitted from a laser plasma X-ray source and a beam emitted from an undulator X-ray source.
In accordance with another aspect of the present invention, there is provided an X-ray reduction exposure apparatus, comprising: an X-ray illumination optical system as discussed above, for illuminating a mask having a pattern; and an X-ray reduction projection optical system for projecting, in a reduced scale, the pattern of the mask, as illuminated, onto the surface of a wafer.
The apparatus may further comprise scanning means for relatively scanningly moving the mask and the wafer relative to said X-ray reduction projection optical system, at a predetermined speed ratio.
In accordance with a further aspect of the present invention, there is provided a method of manufacturing a device by use of an X-ray reduction exposure apparatus as described above.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.